The present invention relates to a method for measuring distance.
In particular, methods for measuring distance with the aid of an ultrasonic echo method are already known, where an ultrasonic signal is generated by an ultrasonic sensor, this signal is reflected by an obstacle, and the reflected signal is received again by a receiver. The received signal is amplified, rectified, and fed to a low-pass filter. The low-pass filtering smooths the received signal. The emitted signal is generally made up of a wave packet having several periods of ultrasonic oscillation from the ultrasonic sensor. This results in divergence of the signal, even when the signal is as concentrated as possible. In order to determine a propagation time as accurately as possible, the time of the signal emission is compared to a maximum of the wave packet received. To this end, the maximum of the low-pass filter output signal, which constitutes an envelope curve for an amplitude function of the signal received, is determined. However, analog components are needed for the low-pass filtering. In this context, it is only possible to reconstruct the envelope in an incomplete manner. Analog low-pass filters that can be produced with a justifiable amount of expenditure have only a low order and result in either the ultrasonic frequency being disadvantageously suppressed or the envelope being widened, depending on the frequency limit set for the low-pass filter. The maximum possible selectivity of an ultrasonic receiver is reduced by both of the effects. Therefore, several closely staggered objects can no longer be detected as separate objects since, in each case, the envelope curves run into each other. The accuracy of the distance measurement is particularly reduced in response to the occurrence of interference signals.
In contrast, the method of the present invention possesses the advantage, that the use of a low-pass filter may be eliminated by sampling the received signal at its frequency or a multiple of its frequency, preferably a whole-number multiple. In this context, the frequency of the received signal approximately corresponds to the frequency of the emitted signal. Therefore, the elimination of an analog low-pass filter increases the maximum possible selectivity, one being able to dispense with analog components. The signal processing may instead be carried out in an advantageous manner, using digital components.
It is particularly advantageous to synchronize the received signal, using a PLL stage (phase-locked loop). By this way, the sampling of the received signal may be precisely synchronized to the acoustic frequency of the received signal. Thus, small frequency shifts, such as those caused by the Doppler effect as a result of movement of the obstacle or the vehicle, may be compensated for. In particular, this allows the waveform to be sampled as precisely as possible at the locations of the maxima.
In addition, it is particularly advantageous to evaluate the maxima, since the absolute maximum of the received signal packet may be determined by comparing the maxima of the received signal. The absolute maximum may be determined at an accuracy given by the frequency of the signal. The propagation time of the signal, and therefore the distance, may be calculated from the time of the absolute maximum.
It is also advantageous to digitize the data of the received signal and transmit these data to an evaluation unit. By this way, receiving units may advantageously be connected by a digital bus system to an evaluation unit, which is situated at a suitable position in the vehicle. In this context, digital data may be reliably transmitted, whereby, in particular, a bus system already present in the vehicle may be utilized. This especially simplifies the connection of a multitude of receiving units to an evaluation unit.
Furthermore, is advantageous to control the PLL using a frequency generator because, by this way, the building-up transient is shortened, since there is, in general, only one phase shift between the received signal and oscillating frequency of the PLL. In this context, it is particularly advantageous to use the frequency generator to generate the signal to be emitted, as well, so that a transmitting and receiving unit only needs one frequency generator.
Furthermore, it is advantageous to rectify the signal and determine the maxima of the rectified signal. Sampling is done at two times the frequency, but this further increases the resolution so that maxima may be ascertained more accurately.